Liquid-crystal displays (LCDs) are widely used in projection displays for large screen televisions and monitors. In these LCD-based projection systems, a high power beam of light is passed through a polarizer before being incident on a LCD panel. The LCD panel controls the polarization of the incident light pixel-by-pixel and directs it towards the corresponding polarizer/analyzer, which then directs light having the proper polarization to a projection lens that projects an image onto a screen.
One particularly successful LCD-based projection system is a WGP-based LCoS microdisplay system, which uses both wire grid polarizers (WGPs) and liquid crystal on silicon (LCoS) panels. This microdisplay system, which has been proven to exhibit both high resolution and high image contrast when compared to other microdisplay technologies such as transmissive liquid crystal (xLCD), digital light processor (DLP), and direct-view LCD, typically uses three or more microdisplay panels (e.g., one for each primary color band) to improve on-screen brightness.
Referring to FIG. 1, a conventional 3-panel WGP-based LCoS microdisplay system is shown. The microdisplay system includes a light source 5, which for example is a high-pressure discharge lamp, and a light rod 7. The light rod 7 homogenizes the cone of light produced by the light source 5 to ensure a spatially uniform light distribution. Optionally, the light rod 7 is a polarization conversion light pipe (PCLP) for producing linearly polarized light. A first lens 8a passes the light from the light pipe 7 to a first folding mirror 9, which directs the light to a first dichroic filter 10. The dichroic filter 10 separates out the blue light from the remaining light, and directs the blue light via second 8b and third 8c lenses, and second 17 and third 16 folding mirrors to a first LCoS display panel 20a. The remaining light, which is transmitted through the dichroic filter 10, is directed via fourth and fifth lenses 8d and 8e and a fourth folding mirror 11 to a second dichroic filter 12. The second dichroic filter 12 separates the remaining light into green and red light, the former of which is directed to a second LCoS display panel 20b and the latter of which passes to a third LCoS display panel 20c. 
Prior to reaching each LCoS display panel 20a, 20b, and 20c, the incident light first passes through a WGP 15, 14, and 13 and a trim retarder compensator 21a, 21b, and 21c, respectively. Each WGP 15, 14, and 13 is a polarizer/analyser formed from a plurality of parallel micro-wires that transmits light having a polarization orthogonal to the direction of the parallel micro-wires and reflects light having a polarization parallel to the direction of the wires (e.g., if the polarizers are designed to pass horizontal or P-polarized light, as illustrated in FIG. 1, the micro-wires will be perpendicular to the plane of FIG. 1). Each LCoS panel 20a, 20b, and 20c alters the polarization of the linearly polarized incident light pixel-by-pixel and reflects the modulated light back to the corresponding WGP 15, 14, and 13. Since each WGP 15, 14, and 13 is orientated at approximately ±45° with respect to the principal direction of light propagation, in addition to serving as a polarizer/analyzer, each WGP 15, 13 and 14 also serves as a beamsplitter for separating the incoming light from the outgoing light by steering or deflecting the light reflected from the each LCoS panel along an output optical path orthogonal to the incoming optical path. More specifically, each WGP 15, 14, and 13 reflects S-polarized light (e.g., polarized light rotated by 90° by pixels in an ON state) to the X-cube 19. The X-cube 19 aggregates (i.e., converges) the image from each of the three color channels and, via the projection lens 18, projects the final image onto a large screen (not shown). Optionally, each color channel further includes a pre-polarizer (not shown) and/or a clean-up analyzer (not shown), which for example, may include one or more WGPs and/or dichroic sheet polarizers.
The trim retarder compensators 21a, 21b, and 21c (herein simply referred to as trim retarders), are compensating elements used to improve the contrast performance level of the microdisplay system, which is otherwise limited by the residual birefringence of the LCoS panels in the dark (e.g., off) state. In particular, each trim retarder 21a, 21b, and 21c introduces a phase retardance that cancels the retardance resulting from the inherent birefringence of the corresponding LCoS panel. The term ‘retardance’ or ‘retardation’, as used herein, refers to linear retardance magnitude as opposed to circular retardance magnitude, unless stated otherwise. Linear retardance is the difference between two orthogonal indices of refraction times the thickness of the optical element. Linear retardance causes a phase difference between two orthogonal linear polarizations, where one polarization is aligned parallel to the extra-ordinary axis of the linear retarder and the other polarization is aligned parallel to the ordinary axis of the linear retarder. In contrast, circular retardance causes a relative phase difference between right- and left-handed circular polarized light.
Linear retardance may be described as either in-plane or out-of-plane retardance. In-plane retardance, expressed as optical path length difference, refers to the difference between two orthogonal in-plane indices of refraction times the physical thickness of the optical element. Out-of-plane retardance refers to the difference of the index of refraction along the thickness direction (z direction) of the optical element and one in-plane index of refraction (or an average of in-plane indices of refraction), times the physical thickness of the optical element. Normal incidence rays in a cone bundle see only in-plane retardance, whereas off-axis rays including oblique rays (i.e. non-normal but along the principal S- and P-planes) and skew rays (i.e. non-normal and incident away from the principal S- and P-planes) experience both out-of-plane retardance and in-plane retardance. Notably, in-plane retardance is not observed for the trivial case of 90° ray angle in the birefringent medium.
In the absence of trim retarders 21a-c, the P-polarized polarized light that illuminates each microdisplay panel in the dark (off) state is slightly elliptically polarized upon reflection due to the residual birefringence of the LCoS panels 20a-c. When the elliptically polarized light, which contains both a P- and an S-component, is transmitted to the corresponding WGP 15, 14, 13, the S component is reflected to the X-cube thus allowing dark state light leakage onto the large screen and limiting the contrast of the projection system.
The trim retarders 21a-c improve the contrast level by providing in-plane retardance that compensates for the retardance resulting from the residual birefringence in the LCoS panels 20a-c. Accordingly, each trim retarder 21a-c typically includes an A-plate component having its slow axis configured at orthogonal azimuthal alignment to the slow axis of the corresponding LCoS panel 20a-c (termed “crossed axes”), while its fast axes is configured at orthogonal azimuthal alignment to the fast axis of the corresponding LCoS panel 20a-c. The terms slow axis (SA) and fast axis (FA), as used herein, refer to the two orthogonal birefringent axes when the linear retardance is measured at normal incidence. Notably, the SA and FA locations change with off-axis illumination as well as reversing the SA/FA roles for a negative out-of-plane retardance component at a large angle of incidence.
Since the slow axes of the trim retarders 21a-c and LCoS panels 20a-c are configured at orthogonal azimuthal orientations, the role of the fast/slow axes switches from the trim retarder 21a-c to the LCoS panel 20a-c for normal incidence light. In other words, light having a specific polarization is alternately delayed more then less, or vice-versa, in the trim retarder 21a-c and the LCoS panel 20a-c, respectively. The net effect is zero relative delay for the incoming polarization, and as a result, an unchanged polarization (i.e., the output light is not elliptically polarized). The corresponding WGP 15, 14, 13 and/or optional clean-up polarizer then rejects the output light so that the dark-state panel light leakage does not appear on the screen. Since the trim retarders 21a-c do not alter significantly the throughput of the panel on-state, the resulting sequential contrast (full on/full off) is excellent.
The operating principle of each trim retarder 21a-c is further illustrated in FIG. 2, with reference to the core optics of a single-channel light engine. These core optics include a pre-polarizer 30, a WGP 31, a trim retarder 32, a vertical aligned nematic (VAN)-mode LCoS panel 33, and a clean-up polarizer (not shown). In operation, unpolarized or partial polarized light output from a prior stage illumination (not shown) is passed through the pre-polarizer 30 to obtain P-polarized light. The light is transmitted through the WGP 31 and its polarization extinction ratio is enhanced. The trim retarder 32 preconditions the incoming P-polarization beam and creates an elliptical output. Ideally, the ellipticity in the polarized light incident onto the LCoS panel 33, which is in a dark (off) state, is undone by the residual panel retardance. The reflected light, after completing a double pass through the VAN-LCoS panel 33 and the trim retarder 32, thus remains P-polarized. The remaining P-polarization component transmitted by the WGP 31 is injected back into the illumination system and is eventually lost.
As discussed above, the trim retarder 32 ideally provides an in-plane retardance that matches the in-plane retardance of the corresponding LCoS panel 33 in the dark state. In practice, however, the in-plane retardance (i.e., A-plate retardance) of both the LCoS panel 33 and the trim retarder 32 tends to vary within each component due to manufacturing tolerances in device thickness and material birefringence control, as well as operational drifts (temperature, mechanical stress etc). As a result, to ensure adequate compensation it is common to provide a higher A-plate retardance in the trim retarder 32 than that exhibited by the LCoS panel 33. For example, a trim retarder with an A-plate retardance of 10 nm (at λ=550 nm) is often provided to compensate for a VAN-mode LCoS exhibiting a 2 nm A-plate retardance (at λ=550 nm) in the dark state.
As is known to those skilled in the art, this mismatch in A-plate value requires offsetting of the optic axis of the trim retarder 32, relative to the nominal crossed axes configuration described above. In other words, the trim retarder is ‘clocked-in’ by rotating its azimuth orientation away from the crossed-axes configuration. For example, see J. Chen, M. G. Robinson and G. D. Sharp, “General methodology for LCoS panel compensation”, SID 04, Digest, pp. 990-993, 2004. FIG. 3, which shows the relative azimuthal orientations of the LCoS panel and the trim retarder slow axes, illustrates how the higher value trim retarder is “clocked” away from the bisector of S- and P-polarization planes, in the adjacent quadrant, by an angle φ. When the slow and fast axes of the VAN-LCoS panel bisect the S- and P-polarization planes, as discussed above, when the LCoS retardance is very small (e.g., <<λ/50), and for a trim retarder A-plate retardance up to a quarterwave, the over-clocked angle, φ, is approximately given by:
  ϕ  ≈                    cos                  -          1                    ⁡              (                  [                                                    Γ                a                            ⁡                              (                LC                )                                      /                                          Γ                a                            ⁡                              (                TR                )                                              ]                )              2  where Γa(TR) is the trim retarder A-plate retardance and Γa(LC) is the LCoS A-plate retardance. Accordingly, the over-clocked angle is about 39° when the LCoS exhibits a 2 mm in-plane retardance and the trim retarder provides about 10 nm of in-plane retardance.
In addition to providing in-plane retardance, the trim retarder 32 is also often required to provide out-of-plane retardance to increase the field of view of the LCoS panel. Out-of-plane retardance compensation is often provided with a C-plate component. While a C-plate does not provide any net retardation for normal-incident rays (i.e., normal incident light is unaffected by the birefringence), rays incident off-axis (i.e., at an angle to the extraordinary axis) experience a net retardation that is proportional to the incident angle. A C-plate is considered to be positive if the retardance increases with angle of incidence and negative if the retardance decreases with angle of incidence. Alternatively, a C-plate is considered to be positive if the retardance the birefringence product Δnd is negative (e.g., if ne−no is negative). Since VAN-mode LCoS panels typically function as +C-plates, it is common for trim retarders to include both an A-plate component for compensating the in-plane retardance (i.e., A-plate retardance) and a −C-plate component for compensating for negative out-of plane retardance (i.e., −C-plate retardance). The resulting trim retarders are conveniently termed A/−C-plate trim retarders.
Optionally, these full function A/−C-plate trim retarders include an O-plate. As is well known to those skilled in the art, an O-plate has both in-plane and out-of-plane retardance. O-plates have been stated to provide improved compensation in various LCD projections systems (e.g., see US Pat. Appl. No. 20040085487 and Lu et al, “An O-plate compensated in-plane switching liquid crystal display”, IEEE J. Displ. Technol., Vol. 2, No. 3, pp. 223, 2006). For clarity, an A-plate is an optical retardation element having its extraordinary axis oriented parallel to the plane of the plate, a C-plate is an optical retardation element having its extraordinary axis oriented perpendicular to the plane of the plate (i.e. parallel to the direction of normally incident light), and an O-plate is an optical element having its extraordinary axis (i.e., its optic axis or C-axis) oriented at an oblique angle with respect to the plane of the plate.
Trim retarders may be fabricated from any material or combination of materials used to form conventional optical retarders (e.g., configured as A-plates, C-plates, and/or O-plates). For example, some possible materials include stretched polymer films such as polyvinylalcohol (PVA) or polycarbonate (PC) films, discotic films, aligned films of liquid crystal polymer (LCP) material, organic foils such as cellulose acetate, birefringent crystals, and dielectric thin films. In general, the selected material(s) should: a) provide a uniform, accurate, and reproducible A-plate retardance, b) provide an accurate and reproducible C-plate retardance profile, and c) be durable under high light flux and high temperature conditions. In addition, these properties should be achievable even when the trim retarder has a relatively low retardance value. For example, trim retarders used to compensate VAN-mode LCoS microdisplay panels are typically required to have an A-plate retardance of less than about 30 nm and a −C-plate retardance of about −100 to −380 nm, at 550 nm wavelength.
Of the above-described optical retarder materials, birefringent crystals are known to be one of the most durable in high light flux and high temperature environments. In addition, a birefringent plate may be cut from a raw birefringent crystal such that its optic axis is parallel to the plane of the plate (i.e., an A-plate), perpendicular to the plane of the plate (i.e., a C-plate), or at an oblique angle with respect to the plane of the plate (i.e., an O-plate). The resulting birefringent plate may then be polished to a predetermined thickness to provide a predetermined retardance (e.g., zero-order quarter-wave A-plate retardance). In addition, in the area of birefringent crystal waveplates, pseudo-zero order retarders are routinely fabricated by crossing optic axes of two birefringent crystal plates. The individual layers may have a positive (e.g., single-crystal quartz or single-crystal magnesium fluoride) or a negative (e.g. calcite crystal) birefringence. This cross-axis arrangement has also been used for fabricating achromatic waveplates utilizing two waveplate elements with appropriate dispersion profiles (such as single-crystal quartz and magnesium fluoride combination).
Despite the fact that birefringent crystals are known for their high durability in high light flux and high temperature environments, their use as trim retarders in low-retardance applications, such as the above-described microdisplay systems (MDPS), has been generally considered less than ideal. In general, this is related to the fact that most birefringent crystals configured either as A-plates or low-tilt O-plates have a relatively high birefringence, and thus need to be extremely thin in order to provide the low retardance values associated with VAN-mode LCoS microdisplay panels (e.g., as a true zero-order retarder). Even with its low birefringence (i.e., Δn˜0.009 at λ=550 nm), the physical thickness of a quartz A-plate would need to be about 1.1 μm to produce a nominal 10 nm trim retarder. With a high birefringence A-plate, such as yttrium vandate (YVO4) (Δn˜0.23 at λ=550 nm), the physical thickness would only be about 43 nm for a nominal 10 nm trim retarder. Clearly, it is not practical to work with plates this thin. In addition, these extremely small thicknesses make it difficult to provide a uniform, accurate, and reproducible A-plate retardance. In particular, it is difficult to meet the desired uniformity specifications (e.g., a couple of percentages), and challenging to target the absolute retardance value, with conventional polishing methods.
One approach of using birefringent crystals as trim retarders in the above-described MDPS is to use multiple-wave retardation. For example, a ten-wave plus 10 nm of retardance provides the same retardation effect as a true zero-order 10 nm retarder at the given center wavelength. For a quartz birefringent plate, the ten-wave plus 10 nm retarder (at λ=550 nm) would be approximately 610 μm thick. While this nominal thickness is much more reasonable to work with, it is still challenging to provide a uniform, accurate, and reproducible A-plate retardance. The precision in the required thickness, to hold for example ±5% net retardance tolerance, is ±0.0064%. This thickness tolerance is no different to targeting a 7 nm ±5% tolerance, requiring a ±30 nm physical thickness tolerance, in this example. In addition, the multi-order retarder may be too dispersive over a wide band contrast compensation.
Yet another approach of using birefringent crystals as a trim retarder in the above-described MDPS is to cascade two crystal plate elements with a predetermined azimuthal offset angle.
Referring to FIG. 4a there is shown an example of a non-orthogonal-angle cascade of two substantially equal-magnitude A-plate retarder elements. The magnitude of the first and second A-plate retarders is given by Γ1 and Γ2, respectively (where Γ1≈Γ2), whereas the azimuthal offset angle is given by φo. The resulting dual-layer retarder functions as an in-plane retarder having its slow-axis aligned at φc azimuthal angle, and having an in-plane retardance Γa given approximately by (Γ1+Γ2)cos(φo). When the two A-plate retarder elements are quartz quarter-wave plates aligned at φo≈88.2° and polished such that Γ1≈Γ2, ≈140.5 nm @ λ=550 nm, the net in-plane retardance of the dual-layer retarder is about 7 nm.
Referring to FIG. 4b there is shown an example of a cascade of two A-plate elements having crossed retarder axes. The magnitude of the first and second A-plate retarders is given by Γ1 and Γ2, respectively, while the azimuthal offset angle φo is equal to 90°. The crossed-axes retarder functions as an in-plane retarder having its slow-axis aligned at φc azimuthal angle, and having an in-plane retardance Γa given approximately by (Γ1−Γ2). In other words, the net in-plane retardance is realized from the magnitude mismatches between the first and second A-plates. When the first A-plate retarder is a quartz A-plate of 207 nm retardance, and the second A-plate retarder is another quartz A-plate of 200 nm retardance, oriented at crossed-axes, the net in-plane retardance is about 7 nm.
In each case, the cascade of two A-plate elements also provides a pseudo −C-plate retardance. FIGS. 5 and 6 illustrate the modeled off-axis retardance of the dual-layer retarder and cross-axes retarder, respectively. More specifically, FIGS. 5 and 6 illustrate plots of linear retardance versus angle of incidence (AOI) in air. The equivalent A/C-plate retarder model (e.g., see K. Tan et al., “Design and characterization of a compensator for high contrast LCoS projection systems,” SID 2005, p. 1810, 2005) is realized with an {ne,no} index pair of {1.65, 1.50} for A-plate and {1.50, 1.65} for C-plate at λ=550 nm. The quartz {ne,no} indices are {1.5552, 1.5459} at λ=550 nm. Referring to FIGS. 5 and 6, the pseudo −C-plate retardance is approximately −84 nm @ λ=550 nm and −200 nm @ λ=550 nm, for the dual-layer retarder and the cross-axes retarder, respectively. With regard to the latter, the pseudo −C-plate element is the common retardance of the two sub-elements (e.g., −200 nm).
The main issue with using either the dual-layer or the crossed-axes retarders for trim retarder applications is that the crystal plate thickness and azimuthal offset angle tolerances are extremely tight. For example, a quartz crystal plate having a birefringence of ˜0.009 requires a thickness tolerance within ±100 nm in order to maintain a ±1 nm of net retardance of each sub-element, whereas the azimuthal offset of the two sub-element should be much less than 0.1° in order to yield a net in-plane retardance with a tight distribution. In addition, the non-90° angle offset of the two sub-elements gives rise to circular retardance at normal incidence.
The modelled linear and circular retardance of the dual-layer retarder are shown in FIG. 7, for a two-layer quartz/quartz retarder having slow-axis aligned at 0.2/88.4° and individual in-plane retardance values of 140.5/140.5 nm. The calculation includes an optical activity (isotropic gyration). Optical activity is a material property that enables the material to rotate plane polarized light. There exists both left- and/or right-hand rotation optical isomers. For a single-crystal quartz plate, the rotation per unit length is approximately 25.4°/mm at 550 nm wavelength (see e.g., see P. Yeh, “Optical waves in layered media,” John Wiley & Sons, New York, p. 210, 1988). The dextro-rotary quartz/quartz dual-layer included positive angle rotation (right-handed with respect to viewing the tail-end of the beam), whereas the levo-rotary quartz/quartz dual-layer had the same layer thicknesses, but included negative angle rotation (left-handed). The levo-rotary quartz/quartz dual-layer was oriented at −0.3/86.9° for the first (closer to light incidence) and second quartz layer, respectively.
As illustrated in FIG. 7, the Δnd product of the linear retardance drops off towards the blue wavelength end, while the corresponding circular retardance increases. For some imager panels, such as VAN-mode LCoS, the on-axis birefringence is substantially linear. The presence of circular retardance results in poor image contrast towards the blue wavelength edge. Comparing the dextro- and levo-rotary quartz/quartz dual-layer retarder, with the first layer substantially parallel to the X-axis and the second layer substantially parallel to the Y-axis, the dextro-rotary device is preferred for having a flatter linear retardance spectrum and reduced circular retardance.
The modelled linear and circular retardance for two quartz A-plates at cross-axes are shown in FIG. 8. Depending on the sense of optical activity, the effective axis of the linear retarder is changed, as well as the sign of the net circular retardance in the compound quartz/quartz retarder.
It would be advantageous to provide a trim retarder including a birefringent crystal, which obviates the above-described challenges.
It would also be advantageous to provide a trim retarder including a birefringent crystal, which is practical to fabricate, and which exhibits reasonable thickness and azimuthal angle tolerances.